Extremes in the weather have a disproportionate impact. Concern that global warming will lead to a more variable climate reinforces political resolve to act on limiting emissions. It also fuels a media obsession that every new example of freak weather is a manifestation of global warming. Is this an overreaction?

First, let me make one thing clear. Asking about extreme weather is not the same as questioning the existence of global warming. Clearly, the Earth has heated up during the 20th century, and part of this change is attributable to human activities. This is a different matter from showing that shorter-term weather fluctuations have become more frequent.

Defining more extreme weather requires careful analysis of meteorological statistics. Studies on a variety of time scales must establish whether recent events fall well outside the range of earlier experience. This calls for lengthy accurate records of, say, high and low temperatures, rainfall amounts and wind speeds. Experts from the meteorological services of Australia, the Netherlands, Britain and the US published an example of this type of analysis in 2002. It examined trends in climatic extremes during the second half of the 20th century, by distilling 3,000 continuous rainfall and temperature records from around the world.

With temperature records, the big change has been to warmer nights with a marked rise in minimum temperatures. This has led to declining nighttime frosts, lengthening of the growing season and a reduction in the extreme range between summer maxima and winter minima. Furthermore, there was no evidence of increasing severe summer heatwaves. This all adds up to a decline in the extreme nature of temperature records in the second half of the 20th century.

What about the European heatwave of 2003: widely seen as a harbinger of warmer world? Analysing Burgundy wine harvest dates since 1370 confirms that the summer of 2003 was by far the hottest summer in over 600 years in central France. But there is no clear trend to warmer summers, and the warmth of the 1990s is matched by comparable periods in the 1380s, 1420s and 1680s.

The number of days with heavy rainfall has increased, although other factors may have contributed to this rise. Conversely, there has been a decline in the number of consecutive dry days around the world; so more droughts may not be a feature of a warmer world.

Analysis of tropical storms tells the same story. While, in principle, warmer tropical oceans should produce more intense storms, in practice, there is no evidence of an upward trend. In the Atlantic, the figures are dominated by a natural variation, which produced more hurricanes between the 1940s and the 1960s, far fewer in the 1970s and 1980s, and a surge since 1995. We have longer records of winter storms across Europe, such as the storm that hit Britain in October 1987, or Lothar, which devastated France in December 1999. Contrary to popular belief, there is no evidence of a sustained rising trend in storminess.

All of this sits uneasily with the longer-term evidence of past climate change. During the past 10,000 years, the climate has been remarkably stable, but, before then, in the last ice age, it was much more extreme. This suggests that in a colder, drier global climate, the weather is far more extreme than now.

What has this to do with global warming? The answer lies in the circulation of the North Atlantic. During the last ice age, it was much less stable. The Gulf Stream switched on or off at the drop of a hat. Computer models of the climate suggest that future warming could lead to an increased influx of freshwater from the melting Greenland icesheet and heavier precipitation over Siberia that could switch the Gulf Stream off, tipping us back into a more variable climate. What is more, the Gulf Stream has slowed down in the past 10 to 15 years, although this could be no more than a natural fluctuation.

Even if warming could trigger sudden, effectively unpredictable changes, we must not exaggerate current weather extremes. Failure to take a balanced approach to these events distorts our priorities. So, next time an extreme event is cited as the result of global warming, look closely at the evidence. As the cost of reducing emissions of greenhouse gases hits our energy bills, more and more people will be asking the same question and whether it is worth the sacrifice.

· William Burroughs's latest book, Climate Change in Prehistory, is published by Cambridge UP. To order a copy for £18.99 with free UK p&p, call Guardian book service on 0870 836 0875 or visit www.guardian.co.uk/bookshop/